A major goal of systems neuroscience is to understand how sensory information guides behavior. The approach in our lab is to study how humans and animals make decisions about ambiguous or uncertain sensory information. The neurobiological framework of decision-making for understanding how the brain makes decisions under these conditions suggests that populations of neurons representing possible outcomes compete with one another, such that the first to reach a decision threshold is taken as the choice. These frameworks further postulate that inhibitory neurons are critical for shaping evolving decisions within a neural substrate that supports decision-making. To date there is limited experimental evidence to support the computational roles of inhibitory neurons in decision-making. The challenge towards validating this framework in experimental preparations has primarily been the availability of tools for targeting and controlling specific neural cell typs. In recent years, rodents have become exemplary models for addressing this technical hurdle. To begin to address the fundamental gap in our understanding about the neural mechanism underlying decision- making, we developed a robust and quantitative behavioral paradigm for studying decision-making in mice. In the decision task, mice make judgments about the overall rate of events occurring within a visual stimulus. We find that mice can make accurate and reliable decisions about abstract visual stimuli, and therefore suitable subjects for studying the neurobiology of decision-making. The goal of the proposed two-year research fellowship training is to understand the functional roles of cortical inhibitory neurons in the context of visual decision-making behavior. Specifically, we will independently manipulate the activity of two major classes of inhibitory neurons, parvalbumin (PV; Aim 1) and somatostatin (SOM; Aim 2) -positive neurons, in the posterior parietal cortex of mice performing a visual decision-making task. We will use behavior and extracellular recordings as readout of the optogenetic manipulation. These experiments will further define the contribution of individual subclasses of interneurons to decision-making behavior and neural activity.